The present application claims priority to Japanese Patent Application No. 10-315691 filed on Nov. 6, 1998, the specification, claims, drawings and summary of which is hereby incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a field emission cold cathode.
2. Description of the Related Art
With respect to current control, a field emission cold cathode can be grouped into two groups. A current is controlled by means of a resistor in one of the groups, and by means of a transistor in the other group. Hereinbelow are explained typical field emission cold cathodes with respect to how a current is controlled and with respect to a current limiting device.
Japanese Unexamined Patent Publication No. 5-47296 has suggested a field emission cold cathode including a current limiting device comprised of a resistor, and emitter cones to which the current limiting device is electrically connected. FIG. 1 illustrates the suggested field emission cold cathode. The illustrated field emission cold cathode is comprised of an electrically conductive substrate 9, an insulating layer 5 formed on the substrate 9, a gate electrode 2 formed on the insulating layer 5, and a plurality of emitter cones 6 formed at a surface of the substrate 9 in an gate opening 1 formed throughout the gate electrode 2 and the insulating layer 5. Each of the emitter cones 6 has a resistor 13 therebelow.
Japanese Unexamined Patent Publication No. 5-144370 has suggested a field emission cold cathode including a current limiting device comprised of a resistor, and a gate electrode electrically connected to the current limiting device. FIGS. 2A and 2B illustrate the suggested field emission cold cathode. The illustrated field emission cold cathode is comprised of an electrically conductive substrate 9, an insulating layer 5 formed on the substrate 9, a gate electrode 2 formed on the insulating layer 5, and a plurality of emitter cones 6 formed at a surface of the substrate 9 in an gate opening 1 formed throughout the gate electrode 2 and the insulating layer 5. The gate electrode 2 is comprised of a highly resistive layer 23, and a lowly resistive layer 22 formed on the highly resistive layer 23 in the form of a mesh.
Japanese Unexamined Patent Publication No. 4-284324 has suggested a field emission cold cathode as illustrated in FIG. 3. The suggested field emission cold cathode is comprised of an electrically conductive substrate 9, an insulating layer 5 formed on the substrate 9, a gate electrode 2 formed on the insulating layer 5, a plurality of emitter cones 6 formed at a surface of the substrate 9 in an gate opening 1 formed throughout the gate electrode 2 and the insulating layer 5, a power-feeding line 7 formed on the insulating layer 5, and a connector 18 electrically connecting the gate electrode 2 to the power-feeding line 7.
Japanese Unexamined Patent Publication No. 5-67441 has suggested a field emission cold cathode including a current limiting device comprised of a transistor. FIG. 4 illustrates the suggested field emission cold cathode. The illustrated field emission cold cathode is comprised of an electrically conductive substrate 9, an insulating layer 5 formed on the substrate 9, a gate electrode 2 formed on the insulating layer 5, a plurality of emitter cones 6 formed at a surface of the substrate 9 in an gate opening formed throughout the gate electrode 2 and the insulating layer 5, and a transistor including a base 19 and an emitter 20, formed in the substrate 9.
Japanese Unexamined Patent Publication No. 10-12128 has suggested a field emission cold cathode including a current limiting device having trenches. FIG. 5 illustrates the suggested field emission cold cathode. The illustrated field emission cold cathode is comprised of an electrically conductive substrate 9 including a current flow limiting region 7 at a surface thereof, an insulating layer 5 formed on the substrate 9, a gate electrode 2 formed on the insulating layer 5, and a plurality of emitter cones 6 formed at a surface of the substrate 9 in an gate opening formed throughout the gate electrode 2 and the insulating layer 5. A plurality of trenches is formed throughout the insulating layer 5 and further into a certain depth of the substrate 9. Each of the trenches is filled with an electrical insulator 8. The trenches filled with the electrical insulator 8 define the current-flow limiting region 7.
Japanese Unexamined Patent Publication No. 6-176686 has suggested a field emission cold cathode including a current limiting device comprised of a transistor. Specifically, the suggested field emission cold cathode is comprised of an emitter array, a power-feeding line for feeding electric power to emitters, and a field effect transistor located between the emitter array and the power-feeding line, and having a source region, a gate electrode, and a drain region.
Japanese Unexamined Patent Publication No. 10-50205 has suggested a field emission cold cathode comprising a semiconductor substrate, a plurality of electrically conductive regions formed in the substrate, a plurality of pillar-shaped cathodes each formed on each of the electrically conductive regions, and electrodes each formed on each of the electrically conductive regions with an insulating layer sandwiched therebetween, and each formed with an opening around each of the cathodes. Each of the cathodes includes an upper layer and a lower layer which have electrical conductivities different from each other to thereby establish pn-junction therebetween. The upper layer of each of the cathodes is electrically isolated from the electrically conductive regions.
Japanese Unexamined Patent Publication No. 10-64407 has suggested a field emission cold cathode including an emitter having a sharpened summit and electrically connected to a cathode electrode, a gate electrode having an opening around the emitter, and a pinch-off resistance having saturated current characteristic and located between the emitter and the cathode electrode.
Japanese Unexamined Patent Publication No. 10-21820 has suggested a field emission cold cathode including a silicon substrate having electrically insulating regions, a resistive layer formed on the electrically insulating regions, an insulating layer formed on the resistive layer, a gate conductive layer formed on the insulating layer, and a plurality of conical cathodes each located in an opening formed throughout the gate conductive layer and the insulating layer above the electrically insulating regions.
As is obvious, a function of limiting a current flow for preventing destruction of a cathode from abnormal discharge is provided to either a gate electrode or an electrically conductive substrate to which an emitter cone is connected. The function of limiting a current flow may be provided by addition of either a resistor or a transistor to a field emission cold cathode.
However, if the function of limiting a current flow is provided by addition of a resistor to a field emission cold cathode, the resultant field emission cold cathode would be accompanied with a problem that it is impossible to drive the field emission cold cathode with high frequency. Furthermore, there would be caused a problem that the number of emitter cones per a unit area would be reduced, if a current limiting device is formed in a gate electrode.
The reason is as follows. When the function of limiting a current flow is provided by addition of a resistor to a field emission cold cathode, it would be possible to arrange a resistor in the vicinity of an emitter cone, as illustrated in FIGS. 2A and 2B, for instance. In such an arrangement, the function of limiting a current flow would start operating immediately after discharge has started. However, if a flow of discharge current is to be limited only by means of a resistor, a resistor would have to have a great resistance. In accordance with the results of experiments conducted by the inventors, the resistor has to have a resistivity of 0.4 xcexa9 cm or greater. If a high voltage is applied across an anode which captures electrons which have been ejected from a cold cathode, the resistor would be required to have a resistivity greater than 0.4 xcexa9 cm. If such a resistor having a high resistance is provided to a cold cathode, electric charges would move slowly in each of the electrodes in a cold cathode, resulting in that the cold cathode can not operate at high frequencies.
In a cold cathode including a current limiting device comprised of a resistor, when discharge occurs between an emitter cone and a gate electrode, a voltage applied between a gate electrode and an electrically conductive substrate is also applied across the resistor. A cold cathode operates at a voltage of tens of volts. Hence, it would be necessary to space the resistor from the voltage by a certain distance in order to protect the resistor from the voltage gradient generated by the voltage between a gate electrode and an electrically conductive substrate. In accordance with the experiments conducted by the inventors, this distance was determined to be equal to or greater than 5 xcexcm.
When a plurality of gate openings are formed in a 10 xcexcmxc3x9710 xcexcm area and a power-feeding line is formed around the gate openings at a distance of 5 xcexcm in a cold cathode, such a cold cathode could have a density at which emitter cones are arranged per a unit area, of 25% relative to the greatest density a cold cathode could have.
Though the above-mentioned density can be increased by enlarging the above-mentioned area, there would be caused significant variance in a distance between gate openings and a power-feeding line, which is not practical for actual use.
A cold cathode including a current limiting device comprised of a transistor is accompanied with a problem that the cold cathode would be destroyed due to a time delay during movement of electric charges for operation of the transistor, before the transistor actually operates.
The reason is as follows. When a current limiting device is comprised of a transistor, electric charges have to move until a transistor starts operation thereof, namely until a depletion layer expands. That is, a current flow is not limited unless electric charges move, which electric charges are accumulated in both electrostatic capacity of a transistor and electrostatic capacity defined by a wiring extending from the cold cathode to the transistor. If those electric charges are converted into heat, in particular, if those electric charges are discharged in a short period of time, the emitter cones located around a site at which the electric charge are discharged will melt. If such melted emitter cones bridge a gate electrode and an electrically conductive substrate, the gate electrode is not properly insulated from the substrate, resulting in that the cold cathode could no longer operate.
In view of the above-mentioned problems, it is an object of the present invention to provide a field emission cold cathode which can avoid destruction thereof due to abnormal discharge occurring between an emitter cone and a gate electrode without a reduction in the density at which emitter cones can be arranged on a substrate.
There is provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, both the substrate and the gate electrode being provided with a function of restricting a current from flowing therein.
There is further provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode being comprised of a first resistive layer and a second resistive layer formed on the first resistive layer, the first resistive layer having a resistivity higher than a resistivity of the second resistive layer, the second resistive layer being composed of metal or compound thereof, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, the emitter cones being grouped into a plurality of groups each of which includes the predetermined number of the emitter cones, the substrate being formed with trenches surrounding each of the groups when viewed in a direction of a normal line of the substrate, the trenches being filled with an electrical insulator.
It is preferable that the second resistive layer overlaps the trenches when viewed in a direction of a normal line of the substrate.
It is preferable that the second resistive layer is spaced away from any one of the openings by 2.5 xcexcm or greater when viewed in a direction of a normal line of the substrate.
It is preferable that the first resistive layer has a resistivity equal to 0.02 xcexa9cm or greater, and preferably smaller than 2 xcexa9cm.
For instance, the second resistive layer may have a resistivity of about 0.002 xcexa9cm.
For instance, the electrical insulator may be composed of boron phospho silicate glass (BPSG).
It is preferable that the trenches are spaced away from each other by about 10 xcexcm.
There is still further provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode being comprised of a first resistive layer and a second resistive layer formed on the first resistive layer, the first resistive layer having a resistivity higher than a resistivity of the second resistive layer, the second resistive layer containing an impurity at a higher concentration than that of the first resistive layer, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, the emitter cones being grouped into a plurality of groups each of which includes the predetermined number of the emitter cones, the substrate being formed with trenches surrounding each of the groups when viewed in a direction of a normal line of the substrate, the trenches being filled with an electrical insulator.
It is preferable that the gate electrode is composed of polysilicon.
There is yet further provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode being comprised of a first resistive layer and a second resistive layer formed on the first resistive layer, the first resistive layer having a resistivity higher than a resistivity of the second resistive layer, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, the emitter cones being grouped into a plurality of groups each of which includes the predetermined number of the emitter cones, the substrate being formed with trenches dividing a surface of the substrate into a plurality of regions in each of which each of the groups is located, the trenches being filled with an electrical insulator, the second resistive layer overlapping a part of the trenches so that the second resistive layer surrounds at least two of the regions when viewed in a direction of a normal line of the substrate.
For instance, the second resistive layer may be composed of metal or compound thereof. As an alternative, the second resistive layer may contain an impurity at a higher concentration than that of the first resistive layer.
There is still yet further provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode being comprised of a first resistive layer and a second resistive layer formed on the first resistive layer, the first resistive layer having a resistivity higher than a resistivity of the second resistive layer, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, the emitter cones being grouped into a plurality of groups each of which includes the predetermined number of the emitter cones, the substrate being formed with trenches dividing a surface of the substrate into a plurality of regions in each of which each of the groups is located, the trenches being filled with an electrical insulator, the second resistive layer surrounding at least one of the regions and intersecting with the trenches outside the regions when viewed in a direction of a normal line of the substrate.
It is preferable that each of the groups of the emitter cones is located at the center of each of the regions.
It is preferable that the regions are grouped into first and second regions wherein the first region is a region in which the emitter cones are located, and the second region is a region which is located adjacent to the first region and in which no emitter cones are located.
There is further provided a field emission cold cathode including (a) an electrically conductive substrate, (b) a plurality of emitter cones formed at a surface of the substrate, (c) a gate electrode, (d) an insulating layer sandwiched between the substrate and the gate electrode, the gate electrode and the insulating layer being formed with a plurality of openings in alignment to each other, the emitter cones being formed in the openings, the emitter cones being grouped into a plurality of groups each of which includes the predetermined number of the emitter cones, the substrate being formed with trenches dividing a surface of the substrate into a plurality of regions in each of which each of the groups is located, the trenches being filled with an electrical insulator, the gate electrode being comprised of opening-connectors each electrically connecting the openings located in alignment with a group of the emitter cones, to one another, a second resistive layer formed on the first resistive layer and arranged so as to surround the opening-connectors, and a resistive line electrically connecting the each of the opening-connectors to the second resistive layer, the opening connectors and the resistive line both having a resistivity higher than a resistivity of the second resistive layer.
It is preferable that the second resistive layer overlaps the trenches when viewed in a direction of a normal line of the substrate.
It is preferable that the resistive line has a resistivity equal to 0.02 xcexa9cm or greater, and preferably smaller than 2 xcexa9cm.
It is preferable that the resistive line has a length equal to 2.5 xcexcm or greater.
It is preferable that the resistive line has a resistance in the range of 10 kxcexa9 to 1 Mxcexa9 both inclusive.
It is preferable that the opening-connectors and the resistive line have the same resistivity.
The advantages obtained by the aforementioned present invention will be described hereinbelow.
As mentioned earlier, the field emission cold cathode in accordance with the present invention is designed to have a function of current limitation in both a gate electrode and an electrically conductive substrate located just below the emitter cones. Hence, the field emission cold cathode can operate at high frequencies and can avoid a reduction in the density at which emitter cones are arranged on a substrate. In addition, even if there occurs abnormal discharge between a gate electrode and an emitter cone due to foreign materials being present between a gate electrode and an emitter cone, degradation of a vacuum in the vicinity of a cold cathode, and ions flying to a cold cathode, the field emission cold cathode would not be destroyed.
The reason is as follows. If discharge occurs between a gate electrode and any one of the emitter cones arranged in a matrix due to the above-mentioned reasons, positive electric charges accumulated on a gate electrode in accordance with an electrostatic capacity defined between a gate electrode and an electrically conductive substrate are concentrated at a site at which the discharge occurred. Since the gate electrode in the field emission cold cathode in accordance with the present invention is formed to be resistive, a peak maximum current is reduced, and the positive electric charges are gradually concentrated to a discharge site. Hence, electric charges are discharged from the emitter cone to the substrate, resulting in that it is possible to prevent the field emission cold cathode from being destroyed immediately after discharge.
Thereafter, a depletion layer expands in a region just below the emitter cone at which discharge occurred, and hence, a current flow is limited in the region. As a result, there is generated a voltage gradient between the electrically conductive substrate and an upper surface of the region. Thus, a voltage of the emitter cone is raised, and accordingly, discharge at the emitter cone is ceased.
That is, the field emission cold cathode in accordance with the present invention makes possible to avoid damage due to discharge of electric charges accumulated at a gate electrode, by virtue of the resistive layers of the gate electrode. In addition, power supply to the gate electrode and the substrate is stopped by a depletion layer which expands in a current limiting region formed in the trenches and prohibits a current from flowing therein. Thus, discharge can be ceased.
The resistive layers formed at the gate electrode suppress a peak current generated at the beginning of discharge. Hence, the resistive layers may be designed to have a resistance smaller than a resistance of a resistor included in a conventional cold cathode as a current limiting device. Accordingly, the field emission cold cathode in accordance with the present invention can operate at higher frequencies than the operating frequencies of a conventional field emission cold cathode including a current limiting device comprised only of a resistor. That is, it is possible to swiftly vary a voltage to be applied between a gate electrode and an electrically conductive substrate, resulting in that it would be possible to swiftly control an amount of electrons to be ejected from the emitter cones.
In abnormal discharge, a voltage between a gate electrode and an electrically conductive substrate is distributed in accordance with impedance of the first resistive layer of the gate electrode and the trenches filled with electrical insulator. That is, there is generated a smaller difference in voltage in the first resistive layer in comparison with a conventional cold cathode including a current limiting device comprised only of a highly resistive layer formed at a gate electrode. Accordingly, a distance between a gate opening and a resistive line on a surface of the gate electrode may be designed to be smaller than the same in a conventional cold cathode including a current limiting device comprised only of a highly resistive layer. Thus, the field emission cold cathode in accordance with the present invention makes it possible to enhance a density at which emitter cones are arranged per a unit area.
In accordance with the present invention, the resistive layers acting as a current limiting device, formed at the gate electrode, may have a resistance equal to about ⅕ of a resistance of a conventional cold cathode having a resistive gate electrode. This means that the field emission cold cathode in accordance with the present invention could operate at a frequency five times greater than a frequency at which a conventional cold cathode operates.
In addition, a distance between a gate opening and a resistive line in the field emission cold cathode in accordance with the present invention may be ;equal to about xc2xd of the same in a conventional cold cathode having a resistive gate electrode. As a result, a density at which emitter cones are arranged on a substrate can be increased by 70% or greater.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.